MTBF: What is it good for?

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Andrew Rowland's short paper on why MTBF may not be a good choice.

See www.nomtbf.com for more material on the perils of MTBF, and what to do about it.

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MTBF: What is it good for?

  1. 1. 1MTBF: What is it Good For?Andrew Rowland, CREI. INTRODUCTIONThe mean time between failure (MTBF) is arguably the most prolific metric in the field of reliability engineering. TheMTBF is used as a metric throughout a product’s life-cycle; from requirements, to validation, to operational assessment.Unfortunately, MTBF alone doesn’t tell us too much.It’s not that MTBF is a bad metric. The problem is MTBF is an incomplete metric and, as an incomplete metric, it doesn’tlend itself to risk-informed decision making. The real problem is not with the MTBF, it is with the implicit assumption thatfailure times are exponentially distributed.In the following discussion, we will look at two examples where the MTBF alone could lead us to bad decision making.II. EXAMPLESTo illustrate how relying on the MTBF can be misleading, let’s look at two examples. In these examples we will assume thefailure times are Weibull distributed. The Weibull distribution is popular in reliability engineering and the exponential is aspecial case of the Weibull. From the literature we know the probability density function and survival (or reliability) functionof the Weibull can be expressed as follows:f(t) =βηtηβ−1e−( tη )βS(t) = e−( tη )βWe also recall that the mean of a Weibull distributed variable can be estimated as:MTBF = ηΓ(1 +1β)In the functions above, η is referred to as the scale parameter and β the shape parameter.A. Example 1Consider three items; Item A, Item B, and Item C. Perhaps the goal is to select one of these items for our design and therequirement is to have a 90 hour MTBF or greater. All three items have an MTBF of 100 hours. So, from a reliabilityperspective, which is the Item to choose?Under the implicit assumption that failure times are exponentially distributed, we might conclude that any of the three isacceptable, reliability-wise. All three satisfy the 90 hours MTBF requirement. However, let’s look a little deeper into the 100hour MTBF and see if we still agree that any of the three is acceptable.Let’s take a look at the reliability over time of each Item. Figure 1 shows the reliability function over 500 hours for each ofthese Items. Clearly, the reliability of these Items is not the same. Given that each Item has an MTBF of 100 hours, whatis the reliability at 100 hours? Table I summarizes the 100 hour reliability for each Item. Once again, we can see a largedifference between the three Items.Another way to compare these three Items is via the hazard, or failure, rate. Figure 2 shows the hazard function for each Item.The “bathtub” curve is a plot of hazard rate versus time. Thus, Figure 2 shows the “bathtub” curve for each Item. Clearly thehazard rate behavior is very different for these Items.
  2. 2. 2Fig. 1. Reliability Functions for Item A, Item B, and Item CTABLE IRELIABILITY AT 100 HOURS FOR ITEM A, ITEM B, AND ITEM CItem R(100)Item A 0.109 (10.9%)Item B 0.367 (36.7%)Item C 0.521 (52.1%)B. Example 2Consider another situation where we have three items; Item D, Item E, and Item F. Presume for a moment that we have all ofthe data used to derive the MTBF statistic for each Item. The first thing we might do is graphically explore the data. Figure3 shows a set of plots commonly used in graphical analysis of survival data for Item D. Let’s look at the histogram in theupper left corner. We see the distribution is heavy-tailed indicating failure times are not exponentially distributed.Compare the histogram in Figure 3 to that in Figure 4 for Item E and Figure 5 for Item F. Clearly the distribution of failurestimes differs amongst these three items. Yet all three items have the same MTBF. Perhaps we need to look a bit closer at the data!Now that we’ve graphically analyzed the data and concluded we may be looking at different populations, we decide to fit thedata to a distribution and estimate the parameters.Our goal, then, is to estimate the value of β and η for each Item. We use the fitdist function from the R [1] package fitdistrplus[2] which uses maximum likelihood to estimate the parameters. The results for these three populations are summarized inTable II. We can see from these results that the populations are not the same, although all three Items satisfy our 90 hoursMTBF requirement.Now that we’re confident we’re dealing with three different populations all with the same MTBF, what is the implication ofselecting one Item over another? Since we fit the data to a Weibull distribution, we know the shape parameter (β) determinesthe region of the “bathtub” curve. With a β < 1, we are in the early life region, a β = 1 puts us in the useful life region, anda β > 1 indicates wearout. In other words, Item D is dominated by early-life failure mechanisms, Item E is by useful lifefailure mechanisms, and Item F by wearout.As we did with the first example, let’s look at the reliability function for these three Items. Figure 6 shows the reliability
  3. 3. 3Fig. 2. Hazard Functions for Item A, Item B, and Item CFig. 3. Item D: Graphical Analysis of Survival Datafunctions. Similar to the first example, we see the reliability functions are not the same as we would expect from ourassessment of Figure 3, Figure 4, and Figure 5.Let’s assume we are interested in the reliability at 50 hours. The reliability at 50 hours for the three Items can be found inTable III. We see a dramatic difference in the reliabilities and, interestingly, the Item with the highest 50 hour reliability is theItem with the lowest MTBF.
  4. 4. 4Fig. 4. Item E: Graphical Analysis of Survival DataFig. 5. Item F: Graphical Analysis of Survival DataWe can also look at plots of the hazard function for these three Items. These hazard functions are plotted in Figure 7 over500 hours. We see different hazard rate behaviors as we expected from our assessment of the β values we estimated earlier.
  5. 5. 5TABLE IIESTIMATED PARAMETERS FOR ITEM D, ITEM E, AND ITEM FItem Eta Beta MTBFItem D 101.42 0.478 220.7Item E 107.73 1.000 107.7Item F 100.84 4.524 92.0Fig. 6. Reliability Functions for Item D, Item E, and Item FTABLE IIIRELIABILITY AT 50 HOURS FOR ITEM D, ITEM E, AND ITEM FItem R(50)Item D 0.490 (49.0%)Item E 0.645 (64.5%)Item F 0.959 (95.9%)III. CONCLUSIONHopefully we’ve come to understand that stating an MTBF value with no other information doesn’t really tell us much aboutthe reliability of an Item. Neither does it tell us if the Item truly satisfies our reliability needs. We saw in one example threeItems with the same MTBF, but most definitely with different reliability behavior.In the second example, we looked at three Items with different MTBF. Once again, we saw the reliability behavior of theseItems were different. In this example we saw the Item with the largest MTBF having a 50 hour reliability almost half that ofthe Item with the lowest MTBF.Without an understanding of the reliability characteristics that is more complete than simply MTBF are we making good,risk-informed decisions? Selecting Item A or Item D, we can expect to see high rates of failure during validation, reliabilitygrowth testing, or, worse yet, early in customer ownership. If we warrant our product, we can expect large warranty costsassociated with Item A or Item D. Given the competing requirements we need to satisfy, we may need to select Item A orItem D. If we only know the MTBF will we put the necessary barriers in place, such as screening, to minimize the risk?At the other end of the “bathtub” curve, if we select Item C or Item F, our validation or reliability growth testing may not test far
  6. 6. 6Fig. 7. Hazard Functions for Item D, Item E, and Item Fenough into wearout to surface failures. Will we develop a preventive maintenance program for these Items to minimize the risk?MTBF is ingrained in the reliability community as well as throughout most companies. It is unlikely that we will ever see theend of MTBF. Ultimately it comes down to us, as reliability engineers, to understand the limitations of MTBF and educatethose around us to it’s shortcomings. If the reliability community gets in lock-step, we can be the tugboats that change theship’s heading.REFERENCES[1] R Development Core Team, R: A Language and Environment for Statistical Computing. Vienna, Austria: R Foundation for Statistical Computing, 2009.[2] Marie Laure Delignette-Muller and Regis Pouillot and Jean-Baptiste Denis and Christophe Dutang, fitdistrplus: help to fit of a parametric distribution tocensored or non-censored data. 2013Andrew Rowland is a Reliability Consultant. He previously worked as a Reliability and Safety Engineer in the aerospace, defense, and civil nuclear industries.Mr. Rowland received a BSEE in 1999 and a MS in Statistics in 2006. He is an American Society for Quality Certified Reliability Engineer, a member ofthe IEEE Reliability Society, and the American Statistical Association. He may be contacted by email at andrew.rowland@reliaqual.com.

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